1. Field of the Invention
The present invention relates to a fixing device which fixes a toner image transferred to a recording sheet, and an image forming apparatus including this fixing device.
2. Description of the Related Art
An electrographic image forming apparatus, such as a copying machine and a printer, includes a fixing device which thermally fixes a toner image transferred to a recording sheet.
Power consumed by the fixing device occupies a large proportion of power consumption of the image forming apparatus, wherefore power saving of the fixing device is essential in promoting energy saving of the image forming apparatus.
For meeting this demand, for example, JP 2003-307964 A discloses a technology which divides a fixing rotating body of the fixing device (cylindrical fixing film) into areas in a main scanning direction to individually heat the respective areas. Each of the areas is determined either as an area where an image is to be formed (image area), or as a non-image area based on image information on images to be formed. The temperature of the fixing rotating body corresponding to the image areas is maintained at a fixing temperature. The temperature of the non-image areas is adjusted to a temperature lower than the fixing temperature.
By setting the temperature of the non-image areas to a temperature lower than the fixing temperature during heating, reduction of power consumption is achievable.
In recent years, the temperature rising speed of the fixing device is increasing, up to a speed as high as 20° C./sec. or higher in some cases, with further reduction of heat capacities of the fixing rotating body and surrounding components for the purpose of further promotion of energy saving.
With increase in the temperature rising speed, a rapid temperature change is produced between the image areas and the non-image areas. This rapid temperature change may cause a problem of crinkling of the recording sheet after fixation.